1. Field of the Invention
The present invention relates to a slurry recycling system and method for a chemical mechanical polishing (hereinafter simply referred to as CMP) apparatus for recycling a slurry used when polishing workpieces such as wafers and the like in a CMP apparatus.
2. Description of the Related Art
FIG. 4 is a schematic view of the configuration of a CMP apparatus known to the inventors.
As shown in FIG. 4, the CMP apparatus is provided with a surface plate 100 on the surface of which is attached a polishing pad 101, and a carrier 200 for holding a wafer W.
The wafer W is pressed against the top of the polishing pad 101 by the carrier 200. In this state, the surface plate 100 and the carrier 200 are caused to rotate relative to each other by virtue of an unillustrated drive means such as an electric motor. Slurry S from a slurry feed apparatus 300 is continuously supplied to the top of the polishing pad 101, so that the precision of polishing and polishing rate of the wafer W can be improved.
The slurry S used for the polishing of the wafer W is for example slurry containing fumed silica for polishing the interlayer insulating film of the wafer W or slurry containing alumina for polishing a metal film. The former is an alkali slurry comprising a predetermined concentration of silica dispersed uniformly in pure water and held at a pH of about "11", while the latter is a slurry comprising an oxidizing agent for oxidizing the metal dissolved in pure water and held at a pH of about "2" to "4".
The slurry is selected depending on whether an interlayer insulating film of the wafer W is to be polished or a metal film is to be polished. Whatever the case may be, a desired polishing rate can be attained by maintaining the concentration of the abrasives of silica etc. and the pH at predetermined values.
The slurry used for polishing in a CMP apparatus, however, has contained silicate ions created and dissolved therein during the polishing and therefore changed in pH from its original or initial state. Further, the concentration of the abrasives after polishing has changed and therefore repeated use thereof is not possible.
In particular, the slurry S after polishing has mixed in it polishing dross of the polishing pad 101, dressing dross, and other large and small foreign matter. If such a slurry S is reused as it were, the wafer W would be scratched and the wafer W could no longer be used.
Therefore, in the past, as shown in FIG. 4, a waste liquid receiver 110 was placed around the surface plate 100. The slurry S discharged due to the centrifugal force of the surface plate 100 was received by the waste liquid receiver 110, then stored in a waste liquid tank 120, and finally completely disposed of.
The slurry S actually used for polishing a wafer W, however, is just about 5 percent of the total amount of slurry S supplied.
Therefore, the large remaining 95 percent of the slurry S which could be reused if the slurry S actually used for the polishing were not mixed in with it is disposed of. An extremely large amount of the slurry S has therefore been wasted.